The development of microprocessor and other integrated circuits introduces new challenges to the development of power converters. In order to reduce the passive component size, and also to meet the stringent transient response requirement, the switching frequency of the power converter will move into the MHz range in the next few years.
At high frequency and low power application, the effect of the gate-driver on the overall converter becomes more significant. As the operating frequency of power converters is raised, the losses associated with driving the power MOSFET rise, which is proportional to the switching frequency. At low power levels the resulting penalty on the overall converter efficiency become significant. On the other hand, as power MOSFET die are increased in size to improve the MOSFET on-resistance, the gate-source capacitance of the MOSFET increases in a proportional manner. Therefore, in low voltage, high current applications, the gate driving loss will also increase when the low on-resistance MOSFETs are chosen to reduce the conduction loss. The gate drive losses can often offset advantages gained by the lower conduction losses.
Hence, lossless gate drive circuits have attracted much attention in recent years. Resonant gate drivers are an efficient alternative to the conventional methods to drive power MOSFETs. Many approaches have already been proposed. Most of them are designed for one single MOSFET and based on L-C resonance techniques. Such as, “A MOS gate drive with resonant transitions,” IEEE PESC'91, pp. 527-532, proposed by D. Maksimovic, “A resonant MOSFET gate driver with efficient energy recovery,” IEEE transaction on power electronics, Vol. 19, No. 2, March 2004, pp. 470-477, proposed by Y. Chen, F. C. Lee, L. Amoroso, H. Wu, “A resonant power MOSFET/IGBT gate driver,” IEEE, APEC'02, pp. 179-185, proposed by I. D. de Vries. A simple DC-DC converter and a transformer are even adopted in some solutions, like “A new lossless power MOSFET driver based on simple DC/DC converters,” IEEE PESC'95, pp. 37-43, proposed by J. Diaz, M. A. Perez, F. M. Linera, F. Aldana, which makes the gate driver too complicated and the energy that can be recovered is also limited. All the above solutions can only recover limited gate driving loss and have no other benefits.
In a synchronous buck converter, the switching loss of the top MOSFET is another restriction which limits the switching frequency can't go up further, as the switching loss is also proportion to the switching frequency.
A resonant gate driver for two MOSFETs in a synchronous buck converter is also proposed by K. Yao and F. C. Lee in “A novel resonant gate driver for high frequency synchronous buck converters,” IEEE Transaction on power electronics, Vol. 17, No. 2, March 2003, pp. 180-186, but the required control signals are difficult to generate, and the coupled inductor is expensive and hard to design. This solution does not reduce the switching loss of the top MOSFET.
di/dt is another serious issue if the switching frequency is to be increased. The MOSFETs may be falsely triggered if the gate driver cannot clamp, or lock, the gate-source voltage of the MOSFET at less than its threshold while the MOSFET should turn off.
The proposed resonant gate drive circuits of the present invention can solve all the above issues. It can reduce the gate driver energy substantially, it can reduce the switching loss of the top MOSFET in synchronous buck converter, and it can also eliminate di/dt problem. It includes several kinds of configuration, so it can be widely used.